Abstract: Multiple myeloma (MM) is the second most prevalent blood cancer affecting approximately 83,367 Americans. In 2014, it is estimated that there will be 24,050 new cases of MM and 11,090 people will die of this disease. High dose chemotherapy in combination with autologous hematopoietic stem cell transplantation (HSCT) is the standard treatment for MM; however, innovative strategies are required to improve treatment outcome as most patients fail to achieve complete remission post-transplant and 55.1% of patients die within 5 years. Cytomegalovirus (CMV) infection is a potentially fatal complication in patients receiving HSCT, but recent evidence suggests that, when properly treated, it can exert beneficial clinical outcomes. CMV reactivation is associated with a markedly reduced risk of relapse in acute myeloid leukemia (AML) patients treated with allogeneic HSCT and while the mechanism is unknown, CMV-mediated alterations in the composition of NK-cell subsets are likely involved. NK-cells also play a critical role in slowing the progression of MM and adoptive transfer of NK-cells has been used as a means of curtailing the growth of MM in humans. One possible mechanism underpinning the beneficial effect of CMV on clinical outcome in AML patients, which might be beneficial in MM patients as well, is the increased frequency of NKG2Cpos/NKG2Aneg NK-cells with CMV. NKG2C is an activating receptor and NKG2A an inhibitory receptor for the non-classical HLA class I molecule HLA-E. Thus, only NKG2Cpos/NKG2Aneg NK-cells are able to effectively kill HLA-Epos target cells. Interestingly, both AML and MM cells highly express HLA-E, which protects them from them patient NK-cells that are mostly NKG2Apos. As a result of the limited ability of patient NK-cells to target their own MM cells, immunotherapeutic procedures that rely on donor NK-cells have been developed. The infusion of allogeneic NK-cells has shown promise as a means of inducing remission in MM patients; however, efficacy is limited by the difficulty of acquiring adequate numbers of alloreactive NK-cells and high expression of NKG2A relative to NKG2C in expanded NK-cells. Thus, new protocols are needed to enhance the anti-MM activity of NK-cells. Our group has shown that latent CMV infection enhances NK-cell activity against HLA-E expressing tumor cell lines (including the MM cell line U266) through an NKG2C-dependent mechanism. In this study, we propose to extend these findings and examine the role of CMV infection and frequency of NKG2Cpos/NKG2Aneg NK-cells on early post-transplant responses in MM patients receiving autologous HSCT. We also intend to show that NKG2Cpos/NKG2Aneg NK-cells with high anti-MM activity can be preferentially expanded from fresh PBMCs and cryopreserved cord blood samples using HLA-E-transfected feeder cells. If successful, our innovative approach could improve NK-cell-based immunotherapy for the treatment of MM.